I. Field of the Invention
The present invention relates to a valve timing control for an internal combustion engine.
II. Description of Related Art
In order to improve engine performance as well as enhance fuel economy, many internal combustion engines utilize a valve timing control to vary the valve timing as a function of the engine operating conditions. Typically, the valve timing control includes a stator body having a plurality of annularly spaced interior cavities. A rotor having a plurality of vanes is disposed within the stator body so that one vane is positioned within each cavity of the stator body. The rotor is rotatable between a first and second rotational position relative to the stator body. Furthermore, the rotor is coupled to the camshaft which controls the opening of the engine valves. Consequently, the timing of the valve openings relative to the crankshaft may be varied by varying the relative rotational position of the rotor relative to the stator body. This relative rotational position of the rotor relative to the stator body is typically controlled hydraulically.
In order to rotatably drive the valve timing control, a sprocket is conventionally attached to the stator body and mechanically coupled to the engine crankshaft via a belt or chain. The sprocket, stator body, and rotor thus rotate in unison with each other except that the angular offset between the stator body and the rotor may be varied.
The valve timing control is subjected to a great deal of mechanical and thermal stress during operation. For example, during engine startup the valve timing control is typically not pressurized with hydraulic fluid. Consequently, the rotor freely rotates relative to the stator body and mechanically impacts the stator body for a short period of time following engine startup.
Since the valve timing control is subjected to both high mechanical and thermal stresses during operation, it has been the previous practice to construct the stator body and rotor from metal. A metal stator body is able to withstand both the mechanical impacts from the rotor as well as the thermal conditions during ordinary operation without undue expansion or warpage.
The use of metal for the valve timing control, however, can disadvantageously increase the overall cost for the valve timing control, but also the weight for the valve timing control.
In order to decrease both the cost and the weight of the valve timing control, there have been previously known efforts to utilize plastic materials for the stator body. These previously known attempts, however, have not proven wholly satisfactory.
More specifically, in one previously known valve timing control, a front plate for the stator housing was replaced with a plastic material. While this achieved some reduction in overall weight and cost for the stator housing, the overall savings in weight and cost were minimal.
The use of inexpensive plastic material for the stator housing has also proven unsuccessful. Such low cost plastic materials are simply unable to withstand the impact from the rotor, especially at engine startup. Furthermore, such plastic materials deform during the elevated temperatures present in the normal operation of the valve timing control. Such deformation of the stator housing for the valve timing control may result not only in destruction of the valve timing control, but potentially engine failure.
There are, however, certain plastic resins that are able to withstand the high temperatures present in the environment of the valve timing control without unacceptable deformation. However, these previously known plastic resins, such as PEEK, are not cost effective and do not have production feasibility.